Sending email servers best practice

If you run an outbound SMTP email server, then there’s a number of things you should be doing to ensure smooth sending of your email. All of the things below are fairly straightforward recommendations that are either specified by RFCs or as general best practice.

1. **Ensure your forward and reverse DNS match -  **

   Also called Forward Confirmed Reverse
   DNS,
   having valid and matching forward and reverse DNS is one of the
   first recommendations in RFC
   1912 (“Make sure
   your PTR and A records match”). It’s a sign that the system
   administrator understands at least the basic
   RFCs. It also
   helps to avoid spoofing of your systems by spammers.

   You need to ensure that the IP address you are testing is the “edge”
   one that your email server connects to other servers with. In most
   cases this is obvious, but you might have a machine with multiple IP
   addresses, or you might be behind some sort of
   NAT
   system, in which case the apparent IP address will be the NAT router
   IP address, so make sure you are testing the right IP address.

   There’s a tool to test that your forward and reverse DNS match
   here. Or you can do
   it easily via Linux command line tools. For instance, here’s the
   forward & reverse DNS for one of our outgoing hosts.

   $ dig +short out1.smtp.messagingengine.com
   66.111.4.25
   $ dig +short -x 66.111.4.25
   out1.smtp.messagingengine.com.

   Note how out1.smtp.messagingengine.com -> 66.11.4.25 and
   66.111.4.25 -> out1.smtp.messagingengine.com, this shows that
   forward and reverse DNS match.

   If you’re using an ADSL connection or similar, then make sure you
   get a static IP (most ADSL providers have this option, it may cost a
   little bit more) and make sure you can get the reverse DNS changed
   (also known as setting a PTR record - your ADSL provider will have
   to do this, and not all offer it, so check with your provider first
   before signing up)

2. Ensure your HELO string matches your reverse DNS

   When your SMTP server sends email, it has to announce it’s name in
   the HELO or EHLO command. Since you have your DNS setup correctly,
   you have a fully-qualified domain name (the reverse DNS name), so
   you can follow RFC
   2821 and use it
   as your HELO/EHLO string:

   The argument field contains the fully-qualified domain name of the
   SMTP client if one is available

   Doing this provides another level of verification that your server
   is who it says it is.

3. Don’t use Sender Address Verification

   At first glance, Sender Address Verification (SAV) seems like a good
   idea. Because SMTP doesn’t include include any intrinsic way to
   authenticate the MAIL FROM address, you just connect to the
   appropriate return host and check if the site will accept email for
   that address.

   Unfortunately SAV creates more problems than it solves. As noted by
   others, it’s easy to work
   around SAV, spammers just send with a valid MAIL FROM address. Given
   they’re already spamming lots of valid addresses, they have lots to
   choose from.

   For spammers that don’t use valid MAIL FROM addresses, the result
   will be that your system ends up looking like it’s attempting to
   attack other systems. For instance, say a spammer sends you 1000
   emails with forged and invalid @fastmail.fm MAIL FROM addresses. To
   check them, your server contacts us 1000 times seeing if you can
   deliver to each address. However that’s exactly the pattern that
   anyone trying to do a dictionary harvest
   attack
   against us would be doing! Without special precautions, your machine
   will now be treated as extremely suspicious because it just tried to
   send to lots of invalid addresses at our server.

If you run an email server and have any more suggestions for this list, let me know at robm@fastmail.fm